This invention relates to a method and apparatus for determining media level in a cassette. A single, fixed media level sensor is located adjacent to where the media is fed from the cassette. The bottom of the cassette is moved up until the top piece of media triggers the fixed sensor. The number of steps of the stepper motor (or the time an analog motor is on) indicates the distance the bottom of the cassette must be raised before the top of the media stack hits the sensor. The number of steps is used directly to determine the height of the stack of media and, therefore, the level of media in the cassette.
It is known, in liquid level measurement devices, to employ a stepper motor. Exemplary of such prior art is U.S. Pat. No. 5,243,860 (""860) to P. Habart, entitled xe2x80x9cLiquid Level Measurement.xe2x80x9d While the (""860) reference employs a stepper motor in which the stepper motor steps are counted in order to determine the level of the contained fluid, the moving sensor position is tracked by a marked belt that is moved by the stepper motor. The stepper motor counts are used to interpolate a level between the marks on the belt. Consequently, a more advantageous system would be presented, then, if the marked belt could be eliminated.
It is also known, in other types of measuring systems, to employ a stepper motor. Exemplary of such prior art is U.S. Pat. No. 4,431,985 (""985) to R. C. Mott, entitled xe2x80x9cStepper Motor System for Digitally Measuring Input Quantities.xe2x80x9d While the (""985) reference employs a stepper motor, the invention set forth in the (""985) reference is based on a relationship of forces. The input quantity results in a force and the stepper motor creates a second force. The relationship of the forces determines the value of the input. In particular, the stepper motor and the device are used for sensing something, which appears to be their only purpose. Conversely, a further advantageous system would be presented, then, if the stepper motor were used to lift and measure the motion of a stack of media.
In devices that have a bin to store sheet media, such as a laser printer paper tray, it is often desired to know the level of the media relative to xe2x80x9cfull.xe2x80x9d Many products currently use a system of photo-interrupt devices and xe2x80x9cflagsxe2x80x9d which activate them. In printers, for example, there are commonly two photo-interrupts for each paper tray. Based on the location of the flag relative to the two interrupts, four different combinations of the flag locations can be generated, namely, off/off, on/off, on/on, and off/on. In turn, the combinations are logically decoded to indicate media level. However, the problems associated with this method of detection are that it does not allow the users to:
1. Accurately know the number of pages left. This is because the system gives a maximum of only four levels i.e., plus or minus xc2xc or more of the media capacity for the tray.
2. Calculate and adjust for thicker/thinner media on-the-fly.
3. Furthermore, it is limited to spatial constraints (size, orientation, and location) of the photo-interrupt sensors and the added cost of these sensors.
Fundamentally, it is desirable for people using printers to know accurately and with high resolution the amount of paper remaining in each tray. The current art is to move an arm past several switches as the level of the paper changes. Resolution is limited by the number of switches. Clearly, more switches cost more money.
It is apparent from the above that there exists a need in the art for an apparatus and method for detecting media level in the cassette that is lightweight through simplicity of parts and uniqueness of structure, and which at least equals the media level detecting characteristics of the known apparatus and methods, but which at the same time employs a method of counting stepper motor steps. It is a purpose of this invention to fulfill this and other needs in the art in a manner more apparent to the skilled artisan once given the following disclosure.
Generally speaking, this invention fulfills these needs by providing an apparatus for detecting media level in a media cassette, comprising a media cassette containing an amount of a media, a media sensing means located at a predetermined distance above the media cassette, a media cassette moving means operatively connected to the media cassette, a drive means operatively connected to the media cassette moving means, and a drive monitoring means operatively connected to the drive means.
In certain preferred embodiments, the media cassette is a paper tray. Also, the media sensing means is a single, fixed sensor. Also, the media cassette moving means is a lift mechanism. Also, the drive means is a stepper motor. Finally, the drive monitoring means employs a counter to count the steps of the stepper motor.
In another further preferred embodiment, the use of the media sensing means, the drive means, and the drive monitoring means provides an accurate measurement of the height of the media located within the media cassette.
The preferred apparatus for detecting media level in a media cassette, according to this invention, offers the following advantages: accuracy in determining the number of pages in a media cassette; good stability; good durability; lightness in weight; improved resolution of media level; and excellent economy. In fact, in many of the preferred embodiments, these factors of accuracy, lightness in weight, resolution, and economy optimized to an extent that is considerably higher than heretofore achieved in prior, known apparatus for detecting media level in a media cassette.